In time-of-flight distance measurement, the time delay between emission and reception of a laser pulse allows for distance calculation. However, when the pulse emission frequency is high and when distances are long, a new pulse could be emitted before the previous pulse has returned to the detector. In this situation it is only possible to make the time of flight measurement if the returned pulse can be correctly matched to an emitted pulse.
FIG. 1 illustrates an example in which there is ambiguity as to which emitted pulse corresponds to which returned pulse.
In this example, a train of laser pulses is emitted at intervals of Ta≈1 μsec, e.g., pulses Pi, Pi+1, Pi+2, Pi+3, Pi+4, Pi+5, Pi+6 Returned pulse Pi is received at time δti returned pulse Pi+1 is received at time δti+1, returned pulse Pi+2 is received at time δti+2, returned pulse Pi+3 is received at time δti+3, returned pulse Pi+4 is received at time δti+4, and returned pulse Pi+5 is received at time δti+5.
If no pulse has been emitted before Pi, there is no ambiguity as to returned pulse Pi as it is detected prior to emission of pulse Pi+1. In the case of pulse Pi the distance di to the target is determined from the one-way time of flight δt/2 of the laser pulse and the speed of light c.
However, returned pulse Pi+1 presents an ambiguity as it is detected after emission of pulse Pi+2 so that it is unknown whether it corresponds to emitted pulse Pi+1 or emitted pulse Pi+2. Similarly, returned pulses Pi+3 and Pi+4 are detected after emission of emitted pulses Pi+2, Pi+3 and Pi+4 so that it is ambiguous as to which returned pulse corresponds to which emitted pulse. Returned pulses Pi+2 and Pi+5 are detected after emission of pulses Pi+5 and Pi+6 and very close in time to one another. Correctly determining time of flight is not possible in these situations.
It can be seen that this problem severely limits the rate at which pulses can be emitted and the range of time-of-flight measurements which can be accommodated. The practical consequence in a device used for point scanning is to limit the scanning rate and the scanning range of the device.
Another problem which arises when scanning in real-world environments is that of variable reflectivity. A non-cooperative or low-cooperative surface may have an albedo of, for example, around 1%. A stainless-steel pipe as in industrial piping or a chromed surface may reflect light at an angle like a mirror such that nearly all or nearly none of the pulse is detected at the scanner. A glass surface may have a very high dynamic range of reflectivity, depending on whether the glass is clean or dirty; if the glass is clean the reflectivity may be very low but if dirty the returned pulse may be easily detected. White paper is a highly cooperative surface with a typical albedo of 90-95%. A surface which is partly black and partly white can result in a deformed returned pulse.
A surface with an albedo of 1% at a range of 100 m is equivalent to an albedo of 100% at 1 km. A cooperative surface allows measurement of distant targets with correspondingly long time-of-flight of the laser pulse. To be paired without any ambiguity to the emitted pulse, a reflected pulse must be detected before the next pulse to be emitted, i.e., the scanning range must be less than the distance corresponding to the time of flight of the emitted pulse period. In this respect a distance of 150 m corresponds to a time of flight on the order of about one μsec. When the scanning speed is small, the distance of ambiguity is large, and as the light returned by a target decreases as the inverse of the square of the distance there may be no returned pulse detected after a delay less than the period of the emitted pulses, and thus no pairing problem arises.
In systems where the range of distances measured is relatively limited, it is possible to emit several laser pulses and open a detection window during an approximate expected arrival time of each returned pulse. This can be done for example where scanning is performed from an aircraft flying at a known height above ground level where the surfaces being scanned are within a limited range. See, for example, U.S. Patent Publication 2008/0192228. The use of detection windows is not workable where the approximate time of arrival of returned pulses cannot be predicted, as in the example of FIG. 1.
Distance measurement using a continuous laser and detecting phase shift of the reflected light has the advantage of high scanning speed, but has the disadvantage that each surface sends its own phase so that detection is a linear combination of the two phases. In a dense industrial environment having for example many pipes, a small pipe can intercept a part of the laser and make the distance to the pipe difficult to distinguish from distance to a surface behind the pipe.
Another issue in scanner design is that of eye safety, which limits the average power of the laser pulses emitted.
An ideal scanner would be capable of very fast scanning over a very wide range of distances, allowing use of a single scanner to measure points indoors as well as outdoors, near and far points relative to the scanner position, and within the laser power constraints required for eye safety. Currently, a user needs one scanner for indoor use and another scanner for outdoor use.
To summarize, issues encountered in scanner design include scanning speed, scanning distance, target albedo, and human eye safety.
Improved methods and apparatus for precise laser distance measurement are desired.